It probably opens and lists people on the left with their details on the right. It looks like a book. Can you group people? It's certainly not obvious that such functionality (which is useful!) exists.
Now click the red bookmark thing on top. Not very intuitive. It looks like a bookmark, not something that switches to a different view. Anyways, viola! The grouping interface! But now I can't see contact details. I need to "flip the page" back to see the details. This is a huge step backwards and is so inefficient that I completely stopped using groups when I moved to Lion.
Now consider Mountain lion: http://6.mshcdn.com/wp-content/gallery/apple-os-x-mountain-l...
Hey! It's the three column view from the old address book! Now I can see my groups, contacts, and details, just like before. Not as skeuomorph though. Now it's just an app that is skinned to look like a book. It doesn't function like a book. There are no pages to flip. It still has the dimensions of a book I guess, but that only makes that left page looks mighty constrained.
UI is about displaying and working with information, and this skeuomorphism is arguably not a very good way of doing either. I can lodge similar complaints about iCal. For instance, why are months always on separate pages? When I'm near the end of a month, I have to keep switching back and forth (with a slow unneeded animation on top). It's a computer, surely we can come up with an interface that discards this old page limitation of paper calendars! Not if you are thinking with skeuomorphism though.